Measurement Device and Associated Method for use in Frequency Selection for Inground Transmission

ABSTRACT

A portable device and associated method are described for use with a system in which a locating signal is transmitted from within the ground during an operational procedure. The locating signal includes a transmission frequency that is selectable from a group of discrete transmission frequencies in a frequency range and the region includes electromagnetic noise that can vary. The portable device includes a receiver having a bandwidth that includes the transmission frequency range and is operable for measuring the electromagnetic noise in the transmission frequency range to establish a frequency content of the electromagnetic noise for use in selecting one of the discrete transmission frequencies that is subsequently transmitted as the locating signal during the operational procedure. The locating signal can be transmitted from a boring tool, a pullback arrangement or an inground cable. A predicted maximum operational depth for a transmitter can be determined prior to the operational procedure.

BACKGROUND OF THE INVENTION

The present invention is generally related to locating and/orcharacterizing the source of an inground transmission frequency and,more particularly, to an apparatus and method for measurement of noisethat may interfere with reception of signals received at the ingroundtransmission frequency.

In certain operations in which a transmitter is moved through theground, substantially continuous location and orientation monitoring ofthe transmitter is necessary. One example of such an operation residesin the use of the transmitter being carried by an underground boringtool. Another example of such an operation resides in moving thetransmitter through a pre-existing borehole or path within the ground.Operations that may use such a pre-existing path include, by way ofexample, the pullback of a utility line through a previously formed borehole and mapping of various types of utility lines including watersupply and waste lines. Conventional locating and monitoring systemsused in conjunction with the foregoing operations are often based onwell-established technology involving the detection of an oscillatingmagnetic field emitted by the transmitter that is moved through theground.

One concern with respect to prior art systems relates to localinterference with the transmitter signal caused by electromagnetic noisethat is present in the environment. The transmitter signal is oftenlimited to a low frequency range of less than 50 kilohertz in order forthe signal to effectively penetrate the ground and be detectable by areceiver located above the surface. Several sources of noise may bepresent in the normal operating conditions of systems that employ atransmitter that is moved in the ground while transmitting at thesefrequencies. For example, underground traffic loop systems, whichautomatically operate stoplights according to the presence ofautomobiles at street intersections, can emit signals in the same lowfrequency range as that used for conventional locator/monitor signals.Another significant source of noise is found in the form of overhead orburied power transmission lines generally emanating noise at 50 Hz or 60Hz (and harmonics thereof). Also, if two or more undergroundtransmitters are operating near one another, the emitted transmittersignals may mutually interfere, thus reducing the accuracy and efficacyof all of the systems involved. Such noise sources, of which theinterfering signal frequencies are known, can be referred to as urbanspecific noise sources. Other sources of low frequency noise may existin the environment, such as those generated by computer networkconnections and community access television (CATV) lines, and these canbe referred to as urban general noise sources.

Urban specific noise and urban general noise sources can limit theaccuracy and the range over which an underground transmitter may beemployed. For instance, the use of the underground transmitter can berestricted under streets with traffic loops. It is recognized byApplicants, however, that the limitations on accuracy and range can befrequency dependent. That is, accuracy and range at one frequency can bemore limited than what is seen at a different frequency in a particularnoise environment.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

Generally, a device and associated method are described for use inconjunction with a system in which a transmitter is moved through theground in a region during an operational procedure while transmitting atransmitter signal having a transmission frequency. The transmissionfrequency is selectable as one of a group of discrete transmissionfrequencies that are spaced apart in a transmission frequency range andthe region includes electromagnetic noise that can vary within theregion and across the transmission frequency range.

In one aspect of the disclosure, the portable device can include areceiver having a receiver bandwidth that at least includes thetransmission frequency range for measuring the electromagnetic noise atleast in the transmission frequency range to establish a frequencycontent of the electromagnetic noise for use in selecting one of thediscrete transmission frequencies as a selected transmission frequencythat is subsequently received by the receiver during the operationalprocedure.

In another aspect of the disclosure, a portable device can include areceiver having a receiver bandwidth that at least includes thetransmission frequency range and the receiver is configured foroperation in (i) a setup mode for measuring the electromagnetic noise atleast in the transmission frequency range to establish a frequencycontent of the electromagnetic noise for use in selecting one of thediscrete transmission frequencies as a selected transmission frequencythat is subsequently transmitted by the transmitter during theoperational procedure and (ii) in a locating mode for receiving theselected transmission frequency to provide certain information relatingto the transmitter.

In still another aspect of the disclosure, a method is described inwhich, prior to the operational procedure, the electromagnetic noise inthe region is detected to generate a set of noise environmentinformation. The set of noise environment information is analyzed toestablish a frequency content of the electromagnetic noise for use inselecting the transmission frequency as one of the plurality of discretetransmission frequencies.

In a further aspect of the disclosure, a portable device and associatedmethod are described for use in conjunction with a system in which anelectromagnetic locating signal is transmitted from within the ground ina region during an operational procedure. The locating signal includes atransmission frequency that is selectable from a group of discretetransmission frequencies that are spaced apart in a transmissionfrequency range and the region includes electromagnetic noise that canvary within the region and across the transmission frequency range. Theportable device includes a receiver having a receiver bandwidth that atleast includes the transmission frequency range and is operable formeasuring the electromagnetic noise at least in the transmissionfrequency range to establish a frequency content of the electromagneticnoise for use in selecting one of the discrete transmission frequenciesas a selected transmission frequency that is subsequently utilized asthe locating signal during the operational procedure.

In a continuing aspect of the disclosure, a portable device is describedfor use in conjunction with a system in which an electromagneticlocating signal is transmitted from within the ground in a region duringan operational procedure. The locating signal includes a transmissionfrequency that is selectable from a group of discrete transmissionfrequencies that are spaced apart in a transmission frequency range andthe region includes electromagnetic noise that can vary within theregion and across the transmission frequency range. The portable deviceincludes a receiver having a receiver bandwidth that at least includesthe transmission frequency range and is configured for operation in (i)a setup mode for measuring the electromagnetic noise at least in thetransmission frequency range to establish a frequency content of theelectromagnetic noise for use in selecting one of the discretetransmission frequencies as a selected transmission frequency that issubsequently utilized as the electromagnetic locating signal during theoperational procedure and (ii) in a locating mode for receiving theselected transmission frequency to provide certain information relatingto the electromagnetic locating signal.

In another aspect of the disclosure an apparatus and associated methodare described for use in conjunction with a system in which atransmitter is moved through the ground in a region during anoperational procedure while transmitting a transmitter signal and theregion includes electromagnetic noise that can vary in frequency andbased on location within the region. Prior to the operational procedure,at least the electromagnetic noise in the region is detected at an aboveground location by a detector. A predicted maximum operational depth ofthe transmitter for reception of the transmitter signal at the aboveground location is determined by a processor based, at least in part, onthe detected electromagnetic noise. The predicted maximum operationaldepth is displayed at least prior to the operational procedure.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be illustrative rather than limiting.

FIG. 1 is a diagrammatic view of one embodiment of a portable devicethat is produced according to the present disclosure, shown here toillustrate its components.

FIG. 2 is a diagrammatic plan view of a region in which an operationalprocedure is to be performed and in which the device of FIG. 1 can beused preparatory to the operational procedure.

FIG. 3 is a graph of noise power versus frequency including plots ofnoise power for three distinct frequencies in the region of FIG. 2 andin vertical alignment with various noise producing features that areshown in FIG. 2.

FIG. 4 is a further simplified diagrammatic illustration of a portion ofthe region of FIG. 2 in a plan view including the intended path with theportable device of FIG. 1 arranged thereabove. The intended path extendsbetween a start point or pit and a stop point or pit. The graph of FIG.3 is shown in alignment with the intended path and in relation to aninground obstacle.

FIG. 5 is a simplified diagrammatic view, in elevation, of the region ofFIG. 4 in which an operator moves the portable device of FIG. 1 along ameasurement path.

FIG. 6 is a flow diagram that illustrates one embodiment for theoperation of the device of FIG. 1 in a noise measurement mode.

FIG. 7 is a screen shot which illustrates one possible appearance of thedisplay screen of the device of FIG. 1 during setup for the noisemeasurement.

FIG. 8 is another screen shot which illustrates one possible appearanceof the display screen of the device of FIG. 1 during the noisemeasurement in which the operator can be instructed to move along themeasurement path and can be given various options to control and monitorthe noise measurements.

FIGS. 9-11 are screen shots which illustrate the possible appearances ofthe display screen of the device of FIG. 1 at three respective positionsalong the measurement path during the noise measurement and where eachfigure illustrates the noise in real time for a respective one of thepositions.

FIG. 12 is another screen shot which illustrates one possible appearanceof the display screen of the device of FIG. 1 subsequent to the noisemeasurement in which the operator can cause the device to enter anAuto-Select Mode.

FIG. 13 is another screen shot which illustrates one possible appearanceof the display screen of the device of FIG. 1 that designates anautomatically identified transmission frequency for subsequent use andin which additional options are provided to the operator.

FIG. 14 is another screen shot which illustrates one possible appearanceof the display screen of the device of FIG. 1 which shows noise valuesthat have been determined for a number of selected frequencies that havebeen monitored along the measurement path.

FIG. 15 is a flow diagram that illustrates another embodiment for theoperation of the device of FIG. 1 in the noise measurement mode.

FIG. 16 is a screen shot which illustrates one possible appearance ofthe display screen of the device of FIG. 1 which shows display outputoptions that may be presented to a user.

FIG. 17 is a screen shot which illustrates one possible appearance ofthe display screen of the device of FIG. 1 responsive to a selection bythe user for the display of measured noise along the intended path atone or more selected frequencies.

FIG. 18 is a screen shot which illustrates one possible appearance ofthe display screen of the device of FIG. 1 responsive to a selection bythe user for the display of a noise map of the operation region.

FIG. 19 a is a process diagram which illustrates baseband decoding for acoherent receiver with coherent demodulation.

FIGS. 19 b and 19 c are waveform diagrams which illustrate plots ofManchester encoded bit 1 and bit 0, respectively, each of which occursin a bit region having a length that corresponds to the time period ofone bit.

FIG. 19 dis a logarithmic plot showing the value of bit error rate,designated as P_(e), plotted against signal to noise ratio, designatedas E_(B)/N_(o).

FIG. 19 e is process diagram that graphically illustrates thedetermination of signal strength at a distance d₀ between a transmitterand receiver.

FIG. 19 f is a process diagram that graphically illustrates thedetermination of a noise value N₀.

FIG. 19 g is a flow diagram which illustrates one embodiment of atechnique for determining maximum usable transmitter depth at pointsalong a borepath or other suitable path in light of detected noise.

FIG. 20 is a screen shot which illustrates one possible appearance ofthe display screen of the device of FIG. 1 responsive to a selection bythe user for the display of maximum usable transmitter depth(s) withinthe operating region.

FIG. 21 is a flow diagram of another embodiment of a method forpredicting maximum usable operational depth for reliable data decoding.

FIG. 22 is a screen shot which illustrates one embodiment of theappearance of the display which provides for confirmation and userselection of transmitter frequencies that are of interest.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments will be readily apparent to those skilled inthe art and the generic principles taught herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown, but is to be accorded the widest scopeconsistent with the principles and features described herein includingmodifications and equivalents, as defined within the scope of theappended claims. It is noted that the drawings are not to scale and arediagrammatic in nature in a way that is thought to best illustratefeatures of interest. Descriptive terminology such as, for example,upper/lower, right/left and the like may be adopted for purposes ofenhancing the reader's understanding, with respect to the various viewsprovided in the figures, and is in no way intended as being limiting.

Turning now to the drawings, wherein like items may be indicated by likereference numbers throughout the various figures, attention isimmediately directed to FIG. 1, which illustrates one embodiment of aportable device, generally indicated by the reference number 10. It isnoted that inter-component cabling has not been illustrated in order tomaintain illustrative clarity, but is understood to be present and mayreadily be implemented by one having ordinary skill in the art in viewof this overall disclosure. Device 10 includes a three-axis antennacluster 11 measuring three orthogonally arranged components of magneticflux indicated as b_(x), b_(y) and b_(z). One useful antenna clustercontemplated for use herein is disclosed by U.S. Pat. No. 6,005,532entitled ORTHOGONAL ANTENNA ARRANGEMENT AND METHOD which is commonlyowned with the present application and is incorporated herein byreference. Antenna cluster 11 is electrically connected to a receiversection 12 which can include amplification and filtering circuitry, asneeded. With regard to the latter, a data detection filter can beprovided as part of the receiver section. The electrical connection tothe receiver section has not been shown, but is understood to bepresent. A tilt sensor arrangement 14 may be provided for measuringgravitational angles from which the components of flux in a levelcoordinate system may be determined. Device 10 further includes agraphics display 16, a receiver section 17, a telemetry arrangement 18having an antenna 19 and a processing section 20 interconnectedappropriately with the various components. The processing section caninclude a digital signal processor (DSP) that is configured to executevarious procedures that are needed during operation. It should beappreciated that graphics display 16 can be a touchscreen in order tofacilitate operator selection of various buttons that are defined on thescreen and/or scrolling can be facilitated between various buttons thatare defined on the screen to provide for operator selection. Such atouch screen can be used alone or in combination with an input device 21such as, for example, a keypad. The latter can be used without the needfor a touch screen. Moreover, many variations of the input device may beemployed and can use scroll wheels and other suitable well-known formsof selection device. The telemetry arrangement and associated antennaare optional. The processing section can include components such as, forexample, one or more processors, memory of any appropriate type andanalog to digital converters. As is well known in the art, the lattershould be capable of detecting a frequency that is at least twice thefrequency of the highest frequency of interest. As one option, a GPS(Global Positioning System) receiver 22 may be included along with a GPSantenna 24. The GPS components may be survey grade in order to provideenhanced position determination accuracy. In one embodiment where a GPSreceiver is not used, some other form of measurement device may beemployed. As one example, shown in phantom, a measuring wheel 28 can besupported on a leg 30 that can be removably attachable with device 10. Asensor 32 is positioned on leg 30 for monitoring rotation of measuringwheel 28 as it is rolled along a surface 34 of the ground. The sensormay be of any suitable type such as, for example, optoelectronic,mechanical or Hall effect with measuring wheel 28 configuredappropriately to cooperate with the selected type of sensor. Sensor 32generates a signal that is monitored by processing section 20 in orderto characterize movement of the device for purposes which will becomeapparent below. Other components (not shown) may be added as desiredsuch as, for example, a magnetometer to aid in position determinationrelative to the drill direction and ultrasonic transducers for measuringthe height of the device above the surface of the ground. In the presentexample, device 10 is configured for serving as a locator for purposesof monitoring and tracking a transmitter that moves through the groundas described in U.S. Pat. No. 6,496,008 (hereinafter the '008 patent)which is commonly owned with the present application and incorporatedherein by reference in its entirety. The '008 patent provides furtherdetails with respect to the components of device 10 and its operationfor purposes of tracking and monitoring a transmitter in the ground. Inthe example of the '008 patent, the transmitter emits a dipole fieldthat is shown in FIGS. 2-4 of the patent. As will be seen, device 10 isfurther configured for use in selecting the frequency at which thetransmitter will subsequently operate in the ground prior to actuallyperforming a particular operational procedure. Such operationalprocedures include, but are not limited to a horizontal directionaldrilling operation to form a borehole, a pullback operation that mightbe performed subsequent to a drilling operation and a survey operationfor mapping a preexisting pathway in the ground. Another type ofoperational procedure that is relevant to these discussions is cablelocating. In cable locating, an underground cable is caused to emit anelectromagnetic field along its length. The locating functionality thatcan be provided by device 10 during the desired operational procedureserves as one mode of the device which may be referred to as a locatingmode.

Irrespective of the particular type of operational procedure that is tobe performed, it should be appreciated that transmitters or sondes canbe made available at different frequencies, but with essentiallyinterchangeable housing outlines. Often, the boring tool or pullbackdevice that operates in the ground is configured for accepting atransmitter having a given housing outline such that the locating signalcould be selected from among a number of available transmitters bysimply installing a transmitter of choice. It would be desirable,however, to provide on-site guidance to operators with respect to whichavailable transmitter would best match a particular operationalprocedure.

Turning to FIG. 2, a region 100 is diagrammatically illustrated in aplan view in which an operational procedure is to be performed. Inparticular, a drill rig 102 is illustrated for use in performing ahorizontal directional drilling procedure to extend a drill string (notshown) along an intended or expected path 110 from a start point 112which is shown as a first pit to stop point 114 which is shown as a stoppit. It should be appreciated that the presence of the start and stoppoints is not necessary to perform the procedure to be described usingportable device 10 and these pits have been shown for illustrativepurposes. The intended path extends beneath a roadway 116 that leads toan intersection 118. A traffic loop 120 is used to control a trafficsignal 122 in a known manner such that traffic loop interference 124 isemitted by the traffic loop. A utility line 130 is buried in theintersection and itself intersects intended path 110 in a plan view. Anoverhead utility power line 136 is positioned proximate to theintersection and emits power line interference 139 which is generallyproduced at either 50 Hz or 60 Hz, and harmonics thereof, depending uponthe physical location of region 100 in the world. It should beappreciated that the measurement of the noise environment shouldpreferably represent the actual noise environment that will beencountered during a subsequent operational procedure such as, forexample, during horizontal directional drilling or a pullback operationfor purposes of installing a utility. In some cases, such as in apullback operation, the expected path is a pre-existing borehole. Theinground path can be pre-existing for other operational procedures suchas, for example, that of mapping an existing utility line.

Referring to FIG. 3, in conjunction with FIG. 2, the former includes agraph 138 of noise power P versus distance along an x axis. Threefrequencies are plotted including 12 KHz which is shown as a solid lineand indicated by the reference number 140, 19 KHz which is shown as adashed line and indicated by the reference number 142, and 33 KHz whichis shown as a line that is made up of pairs of long dashes that areseparated by a short dash and indicated by the reference number 144. Itshould be appreciated that these three frequencies represent actualfrequencies at which a locating signal may be transmitted from aninground transmitter such as one carried by a boring tool; however, thefrequencies are not intended as being limiting and have been selectedfor exemplary purposes. Any suitable transmission frequency may beutilized and considered in accordance with these descriptions.Accordingly, plots in FIG. 3 may be used to represent transmitters thatcorrespond to any available set of transmission frequencies. The plotsof FIG. 3 are shown in a generally vertically aligned relationship withregion 100 of FIG. 2 in order to illustrate the influence thatinterference generating components can have at different frequencies.For example, responsive to traffic loop interference 124, plot 140 at 12KHz, exhibits a peak 150 at a position x₁ having a noise power that isgreater than the noise power that is exhibited at x₁ by plots 142 or144. It is worthwhile to note that peak 150 is at least generallyaligned with utility line 130 of FIG. 2. Outside of peak 150, plot 140exhibits a noise power that is most often the lowest of the three plotsalong the x axis. Responsive to overhead power line noise 139, plot 144exhibits a peak 152 at x₂ that extends significantly above the other twoplots. Plot 144 further exhibits the highest overall noise value alongthe x axis except in the region of peak 150 of plot 140. While plot 142,at 19 KHz does not demonstrate the lowest noise for many positions alongthe x axis, as compared to plot 140, it is notable that plot 142 doesnot include a pronounced peak as do the other two plots.

Turning to FIG. 4, a simplified diagrammatic illustration of region 100is presented that shows start point 112 and stop point 114 with intendedpath 110 extending therebetween in a plan view. A portion of utility 130is shown where it intersects the intended path in this view. Further,portable device 10 is illustrated, arranged generally at one end of theintended path for movement in a direction 151 at least generally alongthe intended path at least as the intended path appears as a projectionat the surface of the ground.

Referring to FIG. 5, a simplified diagrammatic illustration of region100 is presented, in an elevational view, that shows start point 112,stop point 114 and intended path 110 extending therebetween. The surfaceof the ground is indicated by the reference number 152. The intendedpath is configured to pass below utility 130 so as to avoid a collisionor contact with the utility. Portable device 10 is held by an operator154 and moved in the direction of arrow 151 along a measurement path 156that extends to stop point 114. It is noted that the measurement pathcoincides with the intended path in the view of FIG. 4, as a projectionon the surface of the ground. In the present example, operator 154 rollsmeasuring wheel 28 at least generally along a projection of intendedpath 110 at the surface of the ground. As will be further discussed,measuring wheel 28 and leg 30 are not required. That is, other forms ofmeasurement of the movement of portable device 10 may be used such as,for example, GPS receiver 22 or operator 154 may be instructed to movethe portable device at a constant speed in the direction of arrow 151.In another embodiment, an accelerometer arrangement can be used fordetecting movement. For example, in devices such as pedometers, theaccelerometer generates a pulse in response to the movement resultingfrom a footstep, and the resulting pulses are counted to provide anindication of distance. Distance of movement can be rendered moreprecise by combining the accelerometer pulse data with informationregarding the length of an individual operator's stride, as is known theart, although such precision is not considered essential. Directionalmovement can be detected through, for example, the use of magneticsensors using the Earth's magnetic field, as is known in the art. Forpurposes of the present example, movement of device 10 is characterizedwith respect to positions k, k+1 . . . k+n, where the current locationof the portable device is position k and position k+n is located at stoppoint 114.

Turning to FIGS. 6 and 7, the former is a flow diagram which illustratesone embodiment of a method, generally indicated by the reference number200, for the operation of device 10, while the latter figure illustratesscreen 16 of device 10. Initially, at 202 in FIG. 6, device 10 canrequest information concerning the target depth for the intended pathand information concerning the transmitter. In the corresponding screenshot of FIG. 7, operator 154 (FIG. 5) can enter the selected or targetoperating depth. Transmitter power may also be entered. In the presentexample, a depth of 8 feet has been selected at 204 using an input line205. An edit selection 206 can provide for revising any entries on thescreen. An input line 208 provides for the entry of information relatingto available transmitter frequencies. In the present example, theoperator has entered the values of 12 KHz, 19 KHz and 33 KHz, whichcorrespond to the frequencies shown in FIGS. 3 and 4 for illustrativepurposes. Virtually any combination of transmitter frequencies can beentered by the operator. Of course, the current entries can be revisedby selecting edit feature 206, for example, by direct selection on thescreen or by using input device 21 of FIG. 1. Once the operator hasconcluded the entry of data, the operator selects a Start NoiseMeasurement button 210.

In the noise measurement mode, which may be referred to as a setup modein a multimode device, operation proceeds to 220 in FIG. 6. Depending onthe particular embodiment of device 10, the measurement of noise can beaccompanied by monitoring of movement of device 10. That is, the noisemeasurement can be weighted based on movement of device 10. As describedabove, movement monitoring in the present example is provided bymonitoring the rotation of measurement wheel 28 as it is rolled alongthe surface of the ground or by using GPS 22. With regard to theoperation of device 10 according to FIG. 6, it will be seen that noisemeasurements proceed over a series of intervals. Each interval can bequite short, with the time period of the interval being selected basedon the transmitter frequencies that are being monitored. Accordingly, aset of noise environment information is detected for the interval thatcan be used to establish the noise that is present at particularfrequencies and/or across a range of frequencies in a continuous manner.

FIG. 8 illustrates one embodiment of the appearance of screen 16 duringthe measurement mode that can be displayed in conjunction with ongoingoperation according to FIG. 6. This screen instructs the operator tomove along the measurement path and provides a number of options thatcan be in the form of buttons defined on the screen. In one option, theoperator can select a Real Time Display button 230 that will causedevice 10 to display the current noise measurement for the transmitterfrequencies that have been selected by the operator. Such a real timedisplay will be discussed in more detail below. Another option is aPause button 232 which causes processing section 20 to at leastmomentarily stop collecting noise information in the current interval,responsive to an actuation from the user. The pause function isimplemented by step 234 of FIG. 6. Once the pause mode is entered, thisstep causes device 10 to monitor button 232 for another userinteraction. In the pause mode, screen 16 can provide a “PAUSED”indication to the operator which can flash and button 232 can display“RESUME”. At the same time, device 10 can provide an aural indication tobring the operator's attention to the paused status of the device suchas, for example, a periodic beep. Responsive to detection of anotheroperator actuation of button 232, operation in FIG. 6 moves to 240 wherethe noise measurement resumes for the current interval. At 242,responsive to the conclusion of the current noise interval, the measurednoise value can be saved along with movement information, if movementinformation is recorded. It should be appreciated that the noiseinformation and optional movement information can be stored in volatilememory for processing purposes. At 244, the noise information isconverted to the frequency domain to establish the frequency content ofthe electromagnetic noise for the current interval. The frequencycontent can be represented as noise power versus frequency. Theconversion can be performed in a well known manner such as, for example,by using a Fast Fourier Transform (FFT). It should be appreciated thatanother embodiment, yet to be described, does not require the use of atime domain to frequency domain transform. Based on the results of theconversion, at 246, a power spectrum of the noise is established in thefrequency domain for the current interval. This power spectrum can bedisplayed as will be further discussed below. At 248, a noise value canbe established for each frequency of interest. In the present example,the frequencies of interest are 12 KHz, 19 KHz and 33 KHz. At 250, thenoise values for the frequencies of interest can be scaled and saved. At252, the real time noise can be displayed on display 16 of FIG. 1. Whilethe noise values correspond to the current interval, it should beappreciated that the interval duration can be so short that the noisedisplay appears to be continuous at least from a practical standpoint.For example, an interval duration of approximately 0.1 seconds isessentially imperceptible to the operator and provides for monitoringsufficiently low frequencies. Display of the noise information will bedescribed in detail immediately hereinafter for a number of positionsalong the measurement path.

Referring to FIG. 9 in conjunction with FIGS. 2 and 3, display 16 isillustrated for an operator selection of the real time display atposition x₁ of FIG. 3. The display is presented in a bar graph formhaving a horizontal axis 300 that represents frequency and a verticalaxis 302 that represents noise power on a 0-10 scale. A 12 KHz bar 304extends to about 7.1 on the noise scale, a 19 KHz bar 306 extends toabout 3.05 on the noise scale and a 33 KHz bar 308 extends to about 4.8on the noise scale. A peak value 310 for 12 KHz is indicated by anasterisk for bar 304 as well as an average value 312 for 12 KHz which isindicated by a triangle in the body of bar 304. It is noted that peakand average values can be displayed for each frequency although suchvalues have not been shown for the remaining frequencies and in relatedfigures for purposes of illustrative clarity. The peak and averagevalues can be determined in any suitable manner, for example, by usingthe techniques that are described herein. In one embodiment, a plot 316of noise power of the electromagnetic noise versus frequency can bedetermined and display 16 can be configured for illustrating this plot.In the present example, plot 316 represents average noise power althoughpeak noise power is just as readily displayable but has not been shownfor purposes of maintaining illustrative clarity. Thus, the powerspectrum of the noise can be represented in terms of the averagefrequency content, as noise power, plotted against frequency from noisedata obtained during a measurement period. The measurement period cancorrespond to a single interval, as discussed in conjunction with FIG.6, or some combination of intervals with each measurement intervalcontributing a set of noise data to a combined or overall set of noiseenvironment information.

Display 16 of FIG. 9 also includes a first threshold 320 and a secondthreshold 322 which are not required to be shown on the display but havebeen shown for descriptive purposes. It is noted that the specificlevels for the noise thresholds can be based on information that isentered in step 202 of FIG. 6. Such information can include, but is notlimited to, the intended depth for the transmitter during the subsequentoperational procedure as well as the transmission power and frequencyfor each transmitter that is available. It should be appreciated thatcolor can be used to emphasize the noise values in relation to thethresholds, but such color has not been provided due to illustrativeconstraints. In cases where this information is not entered by theoperator, device 10 need not display thresholds. In the present example,the region below first threshold 320 can be considered as a low noiseregion such that a bar that peaks in this region can be presented asgreen in color. The region between first threshold 320 and secondthreshold 322 can be considered as a moderate noise region such that abar that peaks in this region can be presented as yellow in color. Inthis regard, bars 306 and 308 would both be presented in yellow. Theregion above second threshold 322 can be considered as a high noiseregion such that a bar that peaks in this region can be presented as redin color. Accordingly, bar 304 would be presented as red in color. Thevarious noise ranges can be characterized, for example, in the instanceof using a monochrome display, by using hatching within the noise barsor using gray scale values where the shading of the bar corresponds toits associated noise value. It is noted that the pause mode can beentered by the operator using pause mode button 232 in FIG. 9. Moreover,other options can be provided to the operator using buttons on thedisplay screen. Selection of the pause mode can return display 16 to theappearance of FIG. 8.

Referring to FIG. 9, it should be appreciated that display 16 maypresent the real time noise information in a wide variety of ways whileremaining within the scope of the teachings herein. For example, a bargraph format can be modified such that the bars are immediately side byside and each bar includes a numerical frequency designation. Further, abar graph format is not required. In one approach, the display can relyentirely on a numerical presentation which essentially lists eachfrequency and its associated noise value or can use any other suitableform of graphical representation.

FIG. 10 illustrates display 16 for an operator selection of the realtime display at position x₂ of FIG. 3. The 12 KHz bar 304 extends toabout 1.8 on the noise scale and can be green in color based onthreshold 320, 19 KHz bar 306 extends to about 3.2 on the noise scaleand can be yellow in color and 33 KHz bar 308 extends to about 8.9 onthe noise scale and can be red in color.

FIG. 11 illustrates display 16 for an operator selection of the realtime display at position x₃ of FIG. 3. The 12 KHz bar 304 extends toabout 2.9 on the noise scale and can be green in color based onthreshold 320, 19 KHz bar 306 extends to about 3.8 on the noise scaleand can be yellow in color and 33 KHz bar 308 extends to about 5.7 onthe noise scale and can be yellow in color.

Referring to FIGS. 4-6 and 8, upon reaching stop point 114 (FIG. 4), theoperator selects a “Stop Noise Measurement” button 400 (FIG. 8) ondisplay 16. In response, step 402 (FIG. 6) terminates collection ofnoise data. Thereafter, at 404, the average noise power for each of thefrequencies of interest is determined based on the information that isstored for the time intervals that have occurred during movement ofdevice 10 along the measurement path. In performing this determination,movement information measured in optional step 242 can be used to weightthe noise information to account for a lack of movement by the operatorwhich would tend to disproportionately emphasize at least some of theintervals. In the alternative, before the user terminates the noisemeasurement, step 402 causes device 10 to enter the next measurementinterval at 405 and refers operation back to step 220 for the nextmeasurement interval.

Referring to FIGS. 6, 12 and 13, at 406, the operator can be presentedwith an option on screen 16 (FIG. 12) to use an auto-select mode forautomatically choosing one of the frequencies. In order to use theauto-select mode, the user chooses a “Yes” button 410 (FIG. 12).Responsive to this selection, in one embodiment, step 412 of FIG. 6compares the average noise power for each frequency and identifies theone having the lowest value. Step 414 then indicates the selectedfrequency on display 16, for example, as illustrated by FIG. 13, asindicated by the reference number 416. In the present example, theauto-selected frequency is 12 KHz for reasons that will be evident onthe basis of further discussion of the specific values for the averagenoise powers which follows hereinafter. In another embodiment,auto-select can take into account the noise value for each frequency asit relates to thresholds. For example, a frequency having a peak valueat any point along the measurement path exceeding second threshold 322(see FIG. 9, as one example for the 12 KHz frequency) or remaining abovethe second threshold for longer than a predetermined period of time canbe excluded from availability for selection as the transmissionfrequency. As another example, auto-select can favor a particularfrequency for selection as the transmission frequency when theparticular frequency remains below first threshold 320 for a longerperiod of time relative to other frequencies. The frequency selectioncan weight noise behavior relative to the thresholds. For example, thefrequency selection can weight noise behavior relative to firstthreshold 320 as being of more importance than noise behavior relativeto second, upper threshold 322. In other embodiments, the user may markpositions of obstacles and/or other points of interest, for example,using a touch screen display, such that a proposed drill path can bepresented on the display with the designated obstacles and associatednoise values. In this regard, knowing that the noise value for a givenfrequency is above second threshold 322 proximate to an obstacle couldbe a factor in electing not to use that given frequency as thetransmission frequency. On the other hand, exceeding second threshold322 proximate to a pit at an end point of a drill path could be ignored.In any embodiment, the auto-select feature can default to a manualselection mode whenever the selection parameters that are being employeddo not provide a sufficiently determinative result. For example, all ofthe frequencies might have exceeded second threshold 322 and areotherwise relatively close in average noise value.

Display 16 of FIG. 13 provides the operator with the option of switchingto a manual mode using a button 417 which will result in a display ofthe average and peak noise powers for each frequency. Of course, if theoperator wishes to make his or her own decision on the best noise value,the operator can select “No” button 418 which can result in theimmediate display of the average and peak noise powers. The displayscreen in FIG. 13 also provides a button 419 for leaving the noisemeasurement/setup mode and entering the locating mode in a dual modedevice.

Attention is now directed to FIGS. 6 and 14. If the operator chooses notto use the auto-select feature, step 420 causes the generation ofdisplay 16 as it can appear in FIG. 14. Peak noise and average noisepower values are shown for each frequency. The peak noise value for agiven frequency can be determined, for example, based on the measurementinterval that exhibits the highest noise power. Of course, determinationof the peak noise power can be monitored and updated on an ongoingbasis. The peak noise values are shown by flags 501, 502 and 504corresponding to 12 KHz, 19 KHz and 33 KHz, respectively. The peak for12 KHz is at approximately 7.1, the peak for 19 KHz is at approximately3.6 and the peak for 33 KHz is at approximately 8.9. The average noisepower for each frequency is indicated by bars 506, 508 and 510corresponding to 12 KHz, 19 KHz and 33 KHz, respectively. The averagenoise value for 12 KHz is approximately 2.2, the average noise value for19 KHz is approximately 2.8 and the average noise value for 33 KHz isapproximately 5.0. It is of interest to note that, while 12 KHz has thelowest average noise value, the lowest peak noise value is exhibited by19 KHz. In this regard, it may be of interest to the operator to knowwhere each peak noise value occurred along the measurement path. In oneembodiment, device 10 can track this information and indicate such aposition for each noise peak, for example, adjacent to each flag ondisplay 16. In FIG. 14, the x axis position of each peak is indicatedfor 12 KHz, 19 KHz and 33 KHz as 500, 900 and 650 feet, respectively.The operator can compare the locations of these peaks to the locationsof any inground obstacles along the intended path in order to insurethat location accuracy is maintained in the area of the obstacle. In thepresent example, utility 130 (see FIGS. 4 and 5) is located at about 450feet, very near the peak noise value for 12 KHz. In such a situation,the operator might elect to use a transmitter frequency other than 12KHz even though 12 KHz exhibits the lowest average noise power. Theoperator may recall that the real time display at position x₁,illustrated by FIG. 9, showed that 19 KHz exhibited the lowest noiseproximate to obstacle 130. Of course, the operator can return toposition x₁ and repeat the real time noise measurement. As anotherexample, the operator may consider accuracy in approaching stop point114 to be of prime importance. In this case, the operator can elect toselect 12 KHz as the transmitter frequency based on the display atposition x₃ (FIGS. 3 and 11). Of course, the operator may select 12 KHzsolely on the basis of exhibiting the lowest average noise value.

Referring to FIG. 14, it should be appreciated that color coding,hatching and/or shading can be used to emphasize the peak and averagenoise values relative to thresholds 320 and 322 in any suitable manner.For example, average value noise bars 506 and 508 can be green in colorwhile average value noise bar 510 can be yellow. Peak noise flags 501and 504 can be red whereas peak noise flag 502 can be yellow. In oneembodiment, the operator can select a display feature which plots thenoise power for a selected one of the transmitter frequencies againstthe x axis. This plot can appear, for example, in the form of one of theline plots taken from FIG. 4 on display 16.

Attention is now directed to FIG. 15 which is a flow diagram thatillustrates another embodiment of a method, generally indicated by thereference number 200′, for the operation of device 10. It is noted thatmethod 200′ shares a number of steps with method 200 of FIG. 6.Accordingly, descriptions of these shared steps will not be repeated forpurposes of brevity and the reader is referred to the discussions above.What is different, however, resides in the use of a technique in FIG. 15which establishes the noise at one or more given transmitterfrequencies. In one embodiment, a noise measurement is taken, forexample, using a tunable narrowband receiver circuit that issuccessively tuned to each frequency of interest. In other embodiments,digital filter technology can be applied which results in thedetermination of a noise measurement at a discrete frequency, as opposedto a noise determination across a frequency spectrum.

Method 200′ starts with aforedescribed step 202 in which transmitterfrequency, power and target depth information can be entered by theoperator, for example, as described above. At 600, device 10 is set upto receive the first frequency of interest. This can be any of thefrequencies but generally will be either the lowest or the highestfrequency for purposes of simplicity. In the present example, it isassumed that the lowest frequency, 12 KHz, is the first frequency with19 KHz and 33 KHz serving as the second and third frequencies,respectively. Generally, in one embodiment, a discrete Fourier transform(DFT) can be applied to determine the noise that is present at thefrequency of interest. It should be appreciated that any suitabletechnique can be employed including, for example, the Goertzel filteror, as another example, wavelet transformation. At 602, device 10 entersa measurement mode in which noise measurement takes place. Step 234 thenimplements a pause feature that is described above and which causesnoise measurement to suspend and resume responsive to user interactions.At 604, noise measurement and movement monitoring takes place for thecurrent frequency. The measured noise value is saved at 606 along withmovement information for the current frequency. As discussed above,movement information is optional, but can be used to weight the data indetermining average noise values over the extents of the measurementpath. At 610, a decision is made as to whether another frequency is tobe monitored. If that is the case, the frequency is incremented at 612to the next frequency of interest and operation repeats starting at 600for the next frequency as the new current frequency. If, on the otherhand, measurements have been made for all frequencies of interest forthe current interval, operation moves to step 250 which scales the noisevalues and saves them for the current interval. Accordingly, step 606sequentially generates a set of noise environment information whichencompasses all of the frequencies that are of interest. Step 252provides for display of the values from the current interval in the formof a real time display, as described above, to provide the operator withthe opportunity to continuously monitor the noise readings along themeasurement path. Each interval along the measurement path is handled inthis manner until data collection is terminated at 402. The remainder ofthe procedure executes in a manner that is consistent with thedescriptions above. The various presentations on display 16, asdescribed above, are readily implemented using the technique of FIG. 15.

Referring to FIG. 1, in an embodiment where device 10 is operable in thedual modes of noise measurement and locating, it may be advantageous touse the same antenna and receiver circuitry in both modes. That is,antenna 11 and receiver section 17 can be used in both modes. In thisway, it is not necessary to determine the sensitivity of the antenna andreceiver combination as a function of frequency, since the noiseenvironment and the locating signal are measured with the samecomponents. In other words, the sensitivity will be the same for bothmeasurements.

It should be appreciated that device 10 can readily be used for purposesof surveying the noise environment when the operational procedure thatis to be performed is a cable locating procedure. For example, theoperator can use a measurement path that is based on what is thought tobe a projection of the cable onto the surface of the ground. Of course,the operator can enter frequencies that are available for use as thecable locating frequency. It should be appreciated, however, that theapplication of the cable locating frequency can give rise to falselocating signals that will not be present during the noise survey. Onehighly advantageous system and method which essentially eliminates theeffects of false cable locating signals that arise during a cablelocating procedure is described in U.S. Pat. No. 7,151,375 entitledDISTINGUISHING FALSE SIGNALS IN CABLE LOCATING which is commonly ownedwith the present application and incorporated herein by reference in itsentirety.

Turning to FIG. 16, display output options can be provided to theoperator responsive to step 420 of FIGS. 6 and 15. At step 420, screen16 can provide a number of options to the user that are selectable inany suitable manner such as, for example, those described above. Oneselection, for example, is indicated by the reference number 450 andallows the user to choose a bar graph display that can be the display ofFIG. 14.

Referring to FIG. 17, in conjunction with FIG. 16, a selection 452allows the user to select the display of noise power corresponding toone or more individual and previously selected transmitter frequencieswhich can appear, for example, as shown in FIG. 17 with noise powerplotted against distance. Accordingly, a plot of the power spectrum ofthe electromagnetic noise versus distance for each of the selectedfrequencies is provided and displayable.

Turning now to FIG. 18 in conjunction with FIG. 16, another selection454 provides for the display of a noise map 456. Such a noise map caninclude frequency plotted against distance with the noise value at anygiven position on the map being shown using color, gray scale shading orcontour lines that represent constant values of noise power. In thepresent example, contour lines 458 have been used as a result ofillustrative constraints that are associated with the present forum. Alarge area of the map exhibits white noise which is designated atseveral positions using the reference number 460. Noise peaks 462 and464, however, are seen on map 456 such that the user may select atransmitter frequency 470, shown using a dashed line, for an ingroundoperation to be performed in the region corresponding to map 456 whichavoids the noise peaks. FIG. 16 further provides a selection 474 whichallows the user to return to the auto-select mode and may provide thedisplay shown in FIG. 13.

In view of the foregoing, a device is provided for use in conjunctionwith a system in which a transmitter is moved through the ground in aregion during an operational procedure which can involve an undergroundtransmitter that transmits a locating signal or an underground cablethat transmits the locating signal. The signal has a transmissionfrequency that is selectable as one discrete frequency from a group ofdiscrete frequencies. As one example, the selected transmissionfrequency can be chosen based on the availability of sondes that are athand which can be housed in an underground device such as a boring toolor a pullback arrangement wherein each available sonde is configured fortransmitting at a different discrete frequency. As another example, agiven sonde may be tuned or set to transmit at the selected discretetransmission frequency. As yet another example, a given sonde may beconfigured to simultaneously transmit multiple ones of the discretefrequency and a cooperating receiver can be tuned to receive only theselected discrete transmission frequency. In the instance of cablelocating, an above ground transmitter can be configured to cause adesired cable to emit a frequency of interest. Accordingly, in asuitable manner, the transmission frequency can be set to one of aplurality of discrete transmission frequencies that are spaced apart ina transmission frequency range. The region includes electromagneticnoise that can vary within the region and across the transmissionfrequency range. The portable device described herein generally includesa receiver having a receiver bandwidth that at least includes thetransmission frequency range for measuring the electromagnetic noise atleast in the transmission frequency range to establish a frequencycontent of the electromagnetic noise for use in selecting one of thediscrete transmission frequencies as a selected transmission frequencythat is subsequently transmitted during the operational procedure.

It may be desirable to determine and display for the user informationconcerning the maximum usable depth of a transmitter at given pointsalong and/or associated with a borepath or other inground path forreliable data reception, in light of the noise that is present. Oneembodiment of a technique for determining maximum usable depth will bedescribed immediately hereinafter.

Initially, a perfectly coherent receiver is assumed where the carrierhas been coherently demodulated and the bit/symbol timing and the packetsynchronization pattern(s) has been perfectly tracked. The resultingbaseband data can be optimally decoded as shown in the process diagramof FIG. 19 a. The baseband data can be represented mathematically asfollows:

r(t)=S _(i)(t)+v(t); i={0,1}  (1)

where r(t) is the received signal, in voltage, and t is the time inseconds as used throughout the equations presented herein. The functionS_(i)(t) is a Manchester encoded baseband data waveform illustrated inthe diagrammatic plots of FIGS. 19 b and 19 c showing signal amplitudein voltage versus time and in which the waveforms for bit 1 and bit 0are shown, respectively. Although Manchester encoding is used in thecontext of the present descriptions, it should be appreciated that anysuitable type of encoding may be employed. Moreover, it is consideredthat one having ordinary skill in the art can readily apply theteachings brought to light herein to other forms of encoding with thisoverall disclosure in hand.

The transmitted data can be assumed to be corrupted by an Additive WhiteGaussian noise (AWGN), v(t). If the AWGN has a normal distribution (i.e.Gaussian with a mean value of 0) with power α², then the ProbabilityDensity Function (PDF) can be expressed as:

$\begin{matrix}{{f(v)} = \frac{^{\frac{- v^{2}}{2\; \alpha^{2}}}}{\sqrt{2\; \pi \; \alpha^{2}}}} & (2)\end{matrix}$

where:

f=the probability density function (PDF),

v=noise random variable,

e=the exponential function,

α²=power of the noise random variable v (before match filtering).

The Bit-Error-Rate (BER) is a measure of the rate of decoded bits thatare in error. For example, if BER=0.01, then on average, the decoderproduces 1 bit error for every 100 bits it decodes.

Given the conditions above, the probability of decoding a bit in erroris given as:

P(Error)=1−P(Correct)  (3).

Henceforward, the word “Error” may be abbreviated as “E” and the word“Correct” may be abbreviated as “C”. Let y_(k)(T_(B))=γ when the decoderdecodes a bit S_(i). Then the probability of correctly decoding atransmitted bit S_(i) is expressed as:

P(S _(i) |y _(k)=γ)=∫_(L) f _(y)(γ|S _(i))dγ  (4)

where:

y_(k)=the output of the match filter (in volts) as shown in FIG. 19 a.

k=the k^(th) bit of the receiving data sequence.

T_(B)=bit period time (in seconds, see FIGS. 19 b and 19 c)

B=“bit”

L=Limits of integral

γ=y_(k)(T_(B))

d=denotes a derivative operation.

Then, the total probability of correctly decoding the bits is:

$\begin{matrix}{{P(C)} = {{{P( S_{0} )}{\int_{a}^{\infty}{{f_{y}( {\gamma S_{0}} )}\ {\gamma}}}} + {{P( S_{1} )}{\int_{- \infty}^{a}{{f_{y}( {\gamma S_{1}} )}\ {\gamma}}}}}} & (5)\end{matrix}$

where a=0 is appropriate for the baseband waveforms discussed here.Next:

P(S _(i))=Probability that a bit Si was transmitted from thesonde;  (6a)

with i={0,1}

and,

P(S ₀)+P(S ₁)=1  (6b).

Equation (5) can be expressed in terms of the noise v_(k) with a powerof σ² as:

$\begin{matrix}{{P(C)} = {{{P( S_{0} )}{\int_{a}^{\infty}{{f_{v}( {{\gamma - S_{0}}S_{0}} )}\ {\gamma}}}} + {{P( S_{1} )}{\int_{- \infty}^{a}{{f_{v}( {{\gamma - S_{1}}S_{1}} )}\ {\gamma}}}}}} & (7)\end{matrix}$

where:

σ2=noise power after match filtering.

Substituting Equation (2) into Equation (7), and then Equation (7) intoEquation (3), and replacing the “1” in Equation (3) with Equation (6b),one arrives at:

$\begin{matrix}{{P(E)} = {{{P( S_{0} )}( {1 - {\int_{a}^{\infty}{\frac{^{\frac{- {({\gamma - S_{0}})}^{2}}{2\; \sigma^{2}}}}{\sigma \sqrt{2\; \pi}}\ {\gamma}}}} )} + {{P( S_{1} )}{( {1 - {\int_{- \infty}^{a}{\frac{^{\frac{- {({\gamma - S_{1}})}^{2}}{2\; \sigma^{2}}}}{\sigma \sqrt{2\; \pi}}\ {\gamma}}}} ).\mspace{79mu} {Let}}}}} & (8) \\{\mspace{79mu} {{\alpha = ( \frac{\gamma - S_{0}}{\sigma} )}\mspace{79mu} {{and},}}} & ( {9\; a} ) \\{\mspace{79mu} {\beta = {( \frac{\gamma - S_{1}}{\sigma} ).\mspace{79mu} {Then}}}} & ( {9\; b} ) \\{\mspace{79mu} {{{\alpha} = {\frac{1}{\sigma}{\gamma}}}\mspace{79mu} {{and},}}} & ( {10\; a} ) \\{\mspace{79mu} {{\beta} = {\frac{1}{\sigma}{{\gamma}.}}}} & ( {10\; b} )\end{matrix}$

Substituting Equations (9a), (9b), (10a), and (10b) into Equation (8)yields:

$\begin{matrix}{{P(E)} = {{{P( S_{0} )}( {1 - {\int_{\frac{a - S_{0}}{\sigma}}^{\infty}{\frac{^{\frac{- \alpha^{2}}{2}}}{\sqrt{2\; \pi}}\ {\alpha}}}} )} + {{P( S_{1} )}( {1 - {\int_{- \infty}^{\frac{S_{1} - a}{\sigma}}{\frac{^{\frac{- \beta^{2}}{2}}}{\sqrt{2\; \pi}}\ {\beta}}}} )}}} & (11)\end{matrix}$

Let Q(x) denote the tail integral of the Gaussian PDF as:

$\begin{matrix}{{Q(x)}\overset{\Delta}{=}{\int_{x}^{\infty}{\frac{^{\frac{- \theta^{2}}{2}}}{\sqrt{2\; \pi}}\ {\theta}}}} & (12)\end{matrix}$

where:

θ=a Gaussian random variable.

Then Equation (11) can be expressed in terms of Equation (12), asfollows:

$\begin{matrix}{{P(E)} = {{{P( S_{0} )}( {1 - {Q( \frac{a - S_{0}}{\sigma} )}} )} + {{P( S_{1} )}{( {1 - {Q( \frac{S_{1} - a}{\sigma} )}} ).}}}} & (13)\end{matrix}$

Note that:

Q(−x)≡1−Q(x)  (14).

Hence, Equation (13) can be rewritten as:

$\begin{matrix}{{P(E)} = {{{P( S_{0} )}{Q( \frac{S_{0} - a}{\sigma} )}} + {{P( S_{1} )}{{Q( \frac{a - S_{1}}{\sigma} )}.}}}} & (15)\end{matrix}$

Let P(S₀)=P(S₁)=½, and since S₁ ²=S₀ ²=S² are identical (energy-wise),hence S₀=S₁=S. Equation (15) therefore reduces to:

$\begin{matrix}{{P(E)} = {{Q( \sqrt{\frac{1}{2}( \frac{S}{\sigma} )^{2}} )}.{Since}}} & (16) \\{{Q(x)}\overset{\Delta}{=}{\frac{1}{2}{{erfc}( \frac{x}{\sqrt{2}} )}}} & (17)\end{matrix}$

Equation (16) can be re-written, as follows:

$\begin{matrix}{{{P(E)} = {\frac{1}{2}{{erfc}( \sqrt{\frac{S^{2}}{4\; \sigma^{2}}} )}}}{Since}} & (18) \\{{S^{2}\overset{\Delta}{=}{E\{ y_{k}^{2} \}}};{\forall k}} & (19)\end{matrix}$

where E{.} denotes the expected value (i.e. statistical mean of {..}) ofthe random variable inside the bracket, where y_(k) and σ² are definedas:

$\begin{matrix}{y_{k}^{2} = {{\int_{{kT}_{B}}^{{({k + 1})}T_{B}}{( \sqrt{\frac{P_{T}}{T_{B}}} )^{2}\ {t}}} = {\frac{1}{2}P_{T}}}} & ( {20\; a} ) \\{\sigma^{2} = {{\frac{N_{0}}{2}{\int_{- \infty}^{\infty}{{{H(f)}}^{2}\ {f}}}} = {\frac{N_{0}}{2}R_{B}}}} & ( {20\; b} )\end{matrix}$

where H(f) is the Fourier transform of the match filter h(t) which isshown in FIG. 19 a as an integrate and dump. Substituting Equations(20), (21a) and (21b) into Equation (19) yields:

$\begin{matrix}{{P(E)} = {\frac{1}{2}{{erfc}( \sqrt{\frac{1}{4}( \frac{P_{t}T_{B}}{N_{0}} )} )}}} & (21)\end{matrix}$

Equation (22) can be re-written in terms of bit signal-to-noise ratio(E_(B)/N_(o)) as follows:

$\begin{matrix}{{{{P_{e}( \frac{E_{B}}{N_{0}} )} \equiv {P(E)}} = {\frac{1}{2}{{erfc}( \sqrt{\frac{1}{4}\frac{E_{B}}{N_{0}}} )}}}{where}} & ( {22a} ) \\{E_{B} - {P_{t}T_{B}}} & ( {22b} )\end{matrix}$

Equation (22a) gives the BER as a function of bit signal-to-noise ratioE_(B)/N_(o). FIG. 19 dis a plot that shows the BER for a range ofE_(B)/N_(o), based on Manchester encoded OOSK (On-Off-Shift-Keying).Using the plot in FIG. 19 d, one can determine the roll range of thereceiver. For example, to decode the roll packet from a transmitter thathas 7 bits for data (not counting the synchronization bits) one wouldneed to have

$P_{e} < {\frac{1}{7}.}$

The plot in FIG. 19 d shows that one would need

$\frac{E_{B}}{N_{0}} > {3.7\mspace{14mu} {{dB}.}}$

The range for roll data can be estimated under the assumption that thenoise characteristic is the same within the operating radius of thesystem. Furthermore, it is assumed that the amplitude of the transmittedsignal decays by 1/d³ for a dipole transmitting antenna, where d is thedistance between the transmitter and the receiver's antenna. The powerof the signal (S²) is a function of distance, d, as follows:

$\begin{matrix}{{S^{2}(d)} = {( ( \frac{d}{d_{0}} )^{- 3} )^{2}{S_{0}^{2}( d_{0} )}}} & (23)\end{matrix}$

where S₀ ² (d₀) is the value of S₀ ² measured at a distance d₀. UsingEquation (20a), one can re-write Equation (23) as follows:

$\begin{matrix}{P_{T} = {2( \frac{d}{d_{0}} )^{- 6}{S_{0}^{2}( d_{0} )}}} & (24)\end{matrix}$

Using equation (22b) and dividing Equation (24) by N₀, one arrives at:

$\begin{matrix}{\frac{E_{B}(d)}{N_{0}} = {2( \frac{d}{d_{0}} )^{- 6}\frac{{S_{0}^{2}( d_{0} )}T_{B}}{N_{0}}}} & (25)\end{matrix}$

Solving for d yields:

$\begin{matrix}{\hat{d} \leq {{d_{0}( \frac{1}{2} )}^{{- 1}/6}( \frac{E_{B}}{N_{0}} )^{{- 1}/6}( \frac{N_{0}}{{S_{0}^{2}( d_{0} )}T_{B}} )^{{- 1}/6}}} & (26)\end{matrix}$

The variable {circumflex over (d)} in Equation (26) gives the estimatedmaximum distance for decodable roll data as a function of

$\frac{E_{B}}{N_{0}},$

which corresponds to a particular value of BER. Using the exampledescribed earlier: To detect the 7-bit roll packet one will needP_(e)(i.e. BER) to be less than 1/7 for which, according to the chart inFIG. 19 d, one would need to have

$\frac{E_{B}}{N_{0}} > {3.7\mspace{14mu} {{dB}.}}$

To determine {circumflex over (d)} using Equation (26), one would needto know d₀, N₀, and S₀ ² (d₀). To measure d₀ and S₀ ² (d₀), one canplace the transmitter as close to the receiver's antenna as possiblewithout saturating any analog circuitry in the receiver. The value of d₀can be measured directly. Then S₀ ² (d₀) can be evaluated using Equation(19) and Equation (20a). At this short distance, the signal power willbe much stronger than the noise power, so one can neglect the noisepower in the S₀ ² (d₀) measurement. FIG. 19 e is a process diagram thatgraphically illustrates Equations 19 and 20a for estimating S₀ ² (d₀).The quantity N₀ can be determined with the transmitter turned off. FIG.19 f is a process diagram that graphically illustrates how N₀ isdetermined. Also, once the noise power is estimated, if desired, one cansubtract the estimated noise power from estimated noise power in S₀ ²(d₀) that was previously determined as having an influence that isgenerally negligible.

As an example, assume a transmitter is placed 47 inches away from areceiver (i.e. d₀=47 inches) and the receiver measures S₀ ² (d₀)=0.09Volts². Then, the transmitter is turned off and N₀ is measured, forwhich a value of N₀=6.821×10⁻¹² Volt²/Hz is obtained. Assuming that thedistance, {circumflex over (d)}, is one at which the roll packet, whichhas 7 bits can still be decoded

$P_{e} < \frac{1}{7}$

which, from FIG. 19 d, requires

$\frac{E_{B}}{N_{0}} = {3.7\mspace{14mu} {{dB}.}}$

Substituting d₀, N₀, S₀ ²(d₀), and

$\frac{E_{B}}{N_{0}} = {3.7\mspace{14mu} {dB}}$

into Equation (26), the estimated range is {circumflex over (d)}≦1116.9inches (or equivalent=93.1 feet) at which the roll packet can bedecoded.

It is noted that for purposes of the discussion immediately above, aperfectly coherent demodulation of the carrier and perfect knowledge ofthe bit timing and of the packet synchronization is assumed. Further, itis assumed that the detection of the baseband data is performed usingmatch filtering. Any deviation from these assumptions can beaccommodated by using a higher E_(B)/N_(o) value to achieve the sameP_(e) value. In some cases, the system may be too complex to reasonablyanalyze; in that case, one can resort to computer simulation todetermine the BER (i.e. P_(e)) performance as a function of theE_(B)/N_(o) value as shown in FIG. 19 d, and then use Equation (26) toestimate the range of the receiver.

Equation 26 can be used for purposes of determining maximum operationaldepth on an on-the-fly basis by using certain values, as determinedabove, in conjunction with a current noise reading. This can beaccomplished by treating

$( \frac{E_{B}}{N_{0}} )^{{- 1}/6}$

as a constant that is determined with the transmitter and receiverseparated by distance d₀ and substituting the current value for noise asN₀ in the expression

$( \frac{N_{0}}{{S_{0}^{2}( d_{0} )}T_{B}} )^{{- 1}/6}$

while treating the remainder of the expression as a constant with valuesdetermined, as discussed above, with the transmitter and receiverseparated by distance d₀.

With reference to FIG. 19 g, a flow diagram for use in establishing apredicted maximum usable depth is generally indicated by the referencenumber 600. Initially at 602, the predicted operational depthdetermination procedure is initiated, for example, by a user. Theselection to enter procedure 600 for determining this depth may beprovided, for example, as a button 604 in the real time noise displaysof FIGS. 9-11, although this option may be provided to the user at anysuitable time so long as noise data is available to form the basis ofthe analysis. At 610, one or more transmitter frequencies of interestare identified. As one example, the frequencies of 12 KHz, 19 KHz and 33KHz may be identified as potential transmitter frequencies. The user mayidentify that the list of frequencies will remain unchanged forrepetition of this process at subsequent positions. At 612, noise datais collected, for example, using antenna 11 of FIG. 1 in a range thatincludes a current one of the frequencies of interest. In oneembodiment, the noise environment to which antenna 11 is subjected canbe filtered using digital filtering by receiver section 12 to define adetection band at least approximately centered on the current frequencyand sufficiently broad to include encoding of interest such as, forexample, pitch and roll data, among other potential parameters. In oneembodiment, the digital filter can be the data detection filter that isused for purposes of recovering modulated data such as pitch and rolldata during operation as a locator. The data detection filter can becharacterized as having a detection bandwidth that is at leastapproximately centered on the current frequency of interest. In anotherembodiment, this digital filter can have a wider bandwidth than the datadetection filter. It should be noted that the use of digital filtering,as described, does not require the use of a time domain to frequencydomain transform. Because the digital filter includes a filter bandwidththat can be centered on the current frequency, the noise at the currentfrequency and generally within some limited surrounding frequency rangecan be detected. The surrounding range can be relatively narrowed orbroadened as desired. Such digital filtering technology is well known astaught, for example, by Digital Communication Techniques: Signal Designand Detection by Marvin K. Simon, Sami M. Hinedi, and William C.Lindsey, Chapter 4, pages 178-190 (ISBN 0-13-200610-3), which isincorporated herein by reference.

At 614, using Equation (26) and the measured noise value or values asN₀, a predicted maximum value for operational depth can be determined atwhich depth or range the information that is to be encoded on thetransmitter signal will be decodable.

Referring to FIGS. 19 g and 20, having determined the predicted maximumusable operational depth for reliable data reception for one identifiedfrequency of interest, at 618, it is then determined whether anotherfrequency is identified for which the determination is to be made. Ifthe process has been executed for all of the identified frequencies, at620, the information is displayed as shown, for example, in the screenshot of FIG. 20 where the predicted depth for a 12 KHz transmitter is 41feet, the predicted depth for a 19 KHz transmitter is 20 feet and thepredicted depth for a 33 KHz transmitter is 11 feet. At 622, operationcan return, for example, to step 252 of FIG. 6 responsive to selectionof a RESUME button 624 on display 16 of FIG. 20. If, on the other hand,another frequency remains for which the predicted depth determinationhas been requested, at 626, the current frequency is set to the nextidentified frequency and execution returns to step 612 for the newcurrent frequency.

Attention is now directed to FIG. 1 for purposes of describing anotherembodiment for determining the predicted maximum usable operationaldepth for reliable data reception. In this regard, FIG. 1 includes asimulation transmitter 700, having a simulation antenna 702 whichselectively transmits a simulation signal 704 with modulated simulationdata. It should be appreciated that the simulation signal is received byantenna 11 along with any environmental noise. Simulation transmitter700 is configured to transmit signal 704 in a way which mimics orsimulates an actual inground transmitter that is transmitting amodulated signal from a given depth and based on characteristics suchas, for example, calibration constant k, all of which can be specifiedby the user in step 202 of FIG. 6 for each transmitter of interest. Thesimulation antenna can be of the same type as the antenna that is usedby the inground antenna. In the present example, a dipole antenna may beused. In the illustrated embodiment, the simulation antenna is shown ina spaced apart relation from antenna 11. In another embodiment, thesimulation antenna can be co-located with antenna 11, although thisembodiment does not readily admit of illustration and therefore has notbeen shown. The simulation process, like the process of FIG. 19, can beentered, in one embodiment to be described immediately hereinafter, fromreal time noise displays such as those of FIGS. 9-11, by the userselecting MAX DEPTH button 604.

Turning now to FIG. 21, another embodiment of a method for determiningthe predicted maximum usable operational depth for reliable datadecoding is generally indicated by the reference number 720. At 722, theuser can be queried for use in determining the transmitter frequenciesof interest. The screen shot of FIG. 22 illustrates one embodiment forthis query in which the user can confirm that the current specifiedfrequencies are to be used at 724 or if the frequency list is to bemodified at 726. This latter choice can return the screen to the type offrequency selection process that is exemplified by FIG. 7. Further, theuser can select a QUIT button 728 to exit the depth determinationprocedure.

Referring to FIG. 1 in conjunction with FIG. 21, at 730 and after havingidentified the frequencies of interest, the simulation transmitter isset to mimic transmission of the first frequency of interest at aninitial depth and initiates transmission. In one embodiment, the initialdepth can be specified as a shallow depth that is expected to yieldreliable transmission such as, for example, 10 feet or less. At 732, thesimulation signal and noise are received by receiver 12 via antenna 11to form collected data. At 734, processing section 20 subjects thecollected data to the same decode process that is normally used todecode transmissions. At 736, a determination is made as to whether thedata was decodable from modulated simulation signal 704. In oneembodiment, the decoding results are acceptable only if an exactrecovery of the simulated modulation data is accomplished. In otherembodiments, a threshold can be established, for example, in terms ofbit error rate such that the result is acceptable only if the bit errorrate is less than or equal to the threshold value. If the decodingresult is acceptable, at 738, the depth is increased by an increment ΔD.As one example, ΔD can be set as one foot. The process then repeatsbeginning with step 732 and determines if the signal is decodable. Itshould be appreciated that the modulation on the simulation signal mayremain unchanged irrespective of the depth that is being simulated sinceall that is necessary is to test for successful decode of known data.The depth is incremented in this manner until step 736 identifies adepth for which the signal is not decodable. At 740, the depthcorresponding to the last acceptably decodable signal for the currentfrequency is saved. At 744, if another transmission frequency remains tobe simulated, step 746 is entered which changes the simulation signal toreflect the new frequency, based on specified transmitter parameters,and resets the depth to a selected initial shallow value, as discussedabove. The process then repeats until a depth value has been determinedfor each frequency. It should be appreciated that the data that ismodulated on the simulation signal may remain identical irrespective ofother adjustments in the simulation signal such as, for example,changing its signal strength in order to mimic transmission from agreater depth. Further, the same data may likewise be modulated on thesimulation signal for every transmitter frequency that is mimicked.

Referring to FIGS. 20 and 21, at 748, the depth values can be displayedon display 16, for example, as shown in FIG. 20. At 750, execution canbe returned, for example, to step 252 of FIGS. 6 and 15 responsive toselection of RESUME button 624 in FIG. 20.

Although each of the aforedescribed physical embodiments have beenillustrated with various components having particular respectiveorientations, it should be understood that the present invention maytake on a variety of specific configurations with the various componentsbeing located in a wide variety of positions and mutual orientations.Furthermore, the methods described herein may be modified in anunlimited number of ways, for example, by reordering the varioussequences of which they are made up. Accordingly, having described anumber of exemplary aspects and embodiments above, those of skill in theart will recognize certain modifications, permutations, additions andsub-combinations thereof. It is therefore intended that the followingappended claims and claims hereafter introduced are interpreted toinclude all such modifications, permutations, additions andsub-combinations as are within their true spirit and scope.

1. For use in conjunction with a system in which a transmitter is movedthrough the ground in a region during an operational procedure whiletransmitting a transmitter signal having a transmission frequency andsaid transmission frequency is selectable as one of a group of discretetransmission frequencies that are spaced apart in a transmissionfrequency range and said region includes electromagnetic noise that canvary within said region and across said transmission frequency range, aportable device comprising: a receiver having a receiver bandwidth thatat least includes said transmission frequency range for measuring theelectromagnetic noise at least in said transmission frequency range toestablish a frequency content of the electromagnetic noise for use inselecting one of the discrete transmission frequencies as a selectedtransmission frequency that is subsequently received by the receiverduring the operational procedure.
 2. The portable device of claim 1further comprising: a display for displaying the frequency content ofthe electromagnetic noise that is present at least in said transmissionfrequency range.
 3. The portable device of claim 1 wherein said receiverincludes a detector for detecting the electromagnetic noise bymonitoring the electromagnetic noise over a measurement period toproduce a noise signal and a processing section for processing the noisesignal to establish an average frequency content of the electromagneticnoise corresponding to said measurement period.
 4. The portable deviceof claim 3, further comprising: a movement monitoring arrangement forestablishing that the portable device is in one of a state of movementor a stationary state and configured to cooperate with said processingsection to initiate a pause interval by suspending said monitoring ofthe electromagnetic noise responsive to said stationary state and,thereafter, resuming the monitoring of the electromagnetic noiseresponsive to said state of movement such that the electromagnetic noisethat is present during the stationary state has essentially no effect onthe average frequency content.
 5. The portable device of claim 1including a user input arrangement for receiving one or more userinteractions and wherein said receiver is configured to cooperate withthe user input arrangement for accepting a user interaction to initiatea pause interval by suspending measuring of the electromagnetic noiseand, thereafter, to resume the measuring of the electromagnetic noisesuch that the electromagnetic noise that is present during the pauseinterval has essentially no effect on the average frequency content. 6.The portable device of claim 1 wherein said receiver is configured fordetecting the electromagnetic noise over a series of measurement timeintervals and establishing the frequency content of the electromagneticnoise corresponding to each of the measurement time intervals.
 7. Theportable device of claim 1 wherein said receiver is configured forautomatically selecting the selected transmission frequency based on thefrequency content of the electromagnetic noise.
 8. The portable deviceof claim 7 including a display for indicating the selected transmissionfrequency.
 9. The portable device of claim 1 wherein said receiver isconfigured for establishing a power spectrum of said electromagneticnoise over a time interval and including a display that is configuredfor depicting the power spectrum of the electromagnetic noise.
 10. Theportable device of claim 9 wherein said receiver generates a plot ofsaid power spectrum of the electromagnetic noise versus frequency andsaid display is configured for illustrating said plot.
 11. The portabledevice of claim 9 wherein said receiver generates at least one plot ofsaid power spectrum of the electromagnetic noise versus distance for aselected frequency in said transmission range and said display isconfigured for illustrating said plot.
 12. The portable device of claim1 including a user input arrangement for receiving one or more userinteractions and a processor that is configured to cooperate with theuser input arrangement to identify a group of said discrete transmissionfrequencies as chosen in said user interactions.
 13. The portable deviceof claim 12 including a display and wherein said processor is configuredfor deriving a discrete noise level from the measured electromagneticnoise for each transmission frequency within the group of discretetransmission frequencies and for driving said display to indicate anoise level that is associated with each one of the discretetransmission frequencies in the group of discrete transmissionfrequencies.
 14. The portable device of claim 13 wherein said processoris configured for limiting the display to the group of discretetransmission frequencies without displaying noise levels associated withany discrete transmission frequencies not chosen in the userinteractions.
 15. The portable device of claim 1 wherein said receiveris configured for receiving the locating signal during the operationalprocedure and said portable device further includes a locatingarrangement in communication with said receiver for obtaining thetransmitter signal from the receiver during the operational procedurefor use in tracking an inground position of the transmitter.
 16. Theportable device of claim 1 wherein said receiver includes an antennaarray that is configured for detecting the electromagnetic noise alongthree orthogonally opposed receiving axes.
 17. The portable device ofclaim 16 wherein said antenna array is configured for receiving saidtransmitter signal during said operational procedure.
 18. For use inconjunction with a system in which a transmitter is moved through theground in a region during an operational procedure while transmitting atransmitter signal having a transmission frequency and said transmissionfrequency is selectable as one of a group of discrete transmissionfrequencies that are spaced apart in a transmission frequency range andsaid region includes electromagnetic noise that can vary within saidregion and across said transmission frequency range, a portable devicecomprising: a receiver having a receiver bandwidth that at leastincludes said transmission frequency range and configured for operationin (i) a setup mode for measuring the electromagnetic noise at least insaid transmission frequency range to establish a frequency content ofthe electromagnetic noise for use in selecting one of the discretetransmission frequencies as a selected transmission frequency that issubsequently received by the receiver during the operational procedureand (ii) in a locating mode for receiving the selected transmissionfrequency to provide certain information relating to said transmitter.19. A method for use in conjunction with a system in which a transmitteris moved through the ground in a region during an operational procedurewhile transmitting a transmitter signal that is characterized by atransmission frequency that is selectable to set the transmissionfrequency to one of a plurality of discrete transmission frequenciesthat are spaced apart in a transmission frequency range and said regionincludes electromagnetic noise that can vary within said region andacross said transmission frequency range, said method comprising: priorto said operational procedure, detecting the electromagnetic noise insaid region to generate a set of noise environment information; andanalyzing the set of noise environment information to establish afrequency content of the electromagnetic noise for use in selecting saidtransmission frequency as one of said plurality of discrete transmissionfrequencies.
 20. The method of claim 19 including providing a portabledevice for performing said detecting of the electromagnetic noise bymoving the portable device along at least a portion of an above groundpathway that is in a spaced apart relationship with an expected ingroundmovement of the transmitter.
 21. The method of claim 20 wherein theexpected inground movement is defined by an inground pathway that ispreexisting and moving moves the portable device at least generallyalong a projection of the inground pathway onto a surface of the ground.22. The method of claim 20 wherein the expected inground movement isdefined by an inground pathway that is subsequently formed by a boringtool that carries the transmitter such that the inground pathway is anintended path of the boring tool and moving moves the portable device atleast generally along and above the intended path.
 23. The method ofclaim 19 including displaying the frequency content of theelectromagnetic noise that is present at least in said transmissionfrequency range.
 24. The method of claim 19 wherein said detectingproduces a noise signal over a measurement time period and processingthe noise signal produces the set of noise environment information toestablish the frequency content of the electromagnetic noisecorresponding to said measurement period.
 25. The method of claim 19wherein said detecting the electromagnetic noise is performed in aseries of successive time intervals such that a set of noise data isrecorded corresponding to each successive time interval to establish thefrequency content that is associated with each successive time interval.26. The method of claim 25 including configuring a portable device forperforming said detecting and said analyzing such that the portabledevice can be moved along an above ground pathway that is in a spacedapart relationship with an expected inground movement of the transmitterand, as said portable device is moved along at least a portion of theabove ground pathway, initiating said detecting and, when said portabledevice is not moving along said portion of the above ground pathway,pausing said detecting.
 27. The method of claim 25 including configuringa portable device for performing said detecting and said analyzing suchthat the portable device can be moved along an above ground pathway thatis in a spaced apart relationship with an expected inground movement ofthe transmitter and moving the portable device at an at least generallyconstant speed along at least a portion of the above ground pathwayduring said detecting such that the frequency content for each timeinterval is at least approximately equally weighted along that portionof the above ground pathway.
 28. The method of claim 25 includingconfiguring a portable device for performing said detecting and saidanalyzing such that the portable device can be moved along an aboveground pathway that is in a spaced apart relationship with an expectedinground movement of the transmitter and further configuring theportable device for measuring movement along the above ground pathwayand weighting the frequency content for each successive time intervalbased on the measured movement.
 29. The method of claim 19 includingdisplaying said frequency content.
 30. The method of claim 19 includingautomatically electronically selecting one of said discrete transmissionfrequencies as the transmission frequency based on the frequency contentof the electromagnetic noise for use in the operational procedure. 31.The method of claim 19 wherein said analyzing establishes a powerspectrum of said electromagnetic noise over a time period and includingdisplaying the power spectrum of the electromagnetic noise.
 32. Themethod of claim 31 including generating a plot of said power spectrum ofthe electromagnetic noise versus frequency and displaying said plot. 33.The method of claim 31 including generating at least one plot of saidpower spectrum of the electromagnetic noise versus distance for aselected frequency in said transmission range and displaying said plotfor the selected frequency.
 34. The method of claim 19 includingproviding a portable device having a processing section for performingsaid detecting and said analyzing and further providing a user inputarrangement as part of the portable device for receiving one or moreuser interactions and configuring the processing section to cooperatewith the user input arrangement to identify a group of said discretetransmission frequencies as chosen in said user interactions.
 35. Themethod of claim 34 including deriving a discrete noise level from themeasured electromagnetic noise for each transmission frequency withinthe group of discrete transmission frequencies using said processingsection and displaying the discrete noise level that is associated witheach one of the discrete transmission frequencies in the group ofdiscrete transmission frequencies.
 36. The method of claim 35 includinglimiting the display to the group of discrete transmission frequencieswithout displaying noise levels associated with any discretetransmission frequencies not chosen in the user interactions.
 37. Themethod of claim 19 including providing a portable device for performingsaid detecting and said analyzing and further configuring said portabledevice with a locating arrangement for receiving the transmitter signalduring the operational procedure and tracking an inground position ofthe transmitter on the inground pathway.
 38. The method of claim 19including detecting the electromagnetic noise along three orthogonallyopposed receiving axes.
 39. The method of claim 38 including using anantenna array to detect the electromagnetic noise along the threeorthogonal receiving axes and, thereafter, using that antenna array toreceive the transmitter signal during said operational mode.
 40. For usein conjunction with a system in which an electromagnetic locating signalis transmitted from within the ground in a region during an operationalprocedure, said locating signal including a transmission frequency thatis selectable from a group of discrete transmission frequencies that arespaced apart in a transmission frequency range and said region includeselectromagnetic noise that can vary within said region and across saidtransmission frequency range, a portable device comprising: a receiverhaving a receiver bandwidth that at least includes said transmissionfrequency range for measuring the electromagnetic noise at least in saidtransmission frequency range to establish a frequency content of theelectromagnetic noise for use in selecting one of the discretetransmission frequencies as a selected transmission frequency that issubsequently utilized as the locating signal during the operationalprocedure.
 41. A method for use in conjunction with a system in which anelectromagnetic locating signal is transmitted from within the ground ina region during an operational procedure, said locating signal includinga transmission frequency that is selectable from a group of discretetransmission frequencies that are spaced apart in a transmissionfrequency range and said region includes electromagnetic noise that canvary within said region and across said transmission frequency range,said method comprising: configuring a receiver to include a receiverbandwidth that at least includes said transmission frequency range formeasuring the electromagnetic noise at least in said transmissionfrequency range to establish a frequency content of the electromagneticnoise for use in selecting one of the discrete transmission frequenciesas a selected transmission frequency that is subsequently utilized asthe locating signal during the operational procedure.
 42. For use inconjunction with a system in which an electromagnetic locating signal istransmitted from within the ground in a region during an operationalprocedure, said locating signal having a transmission frequency that isselectable from a group of discrete transmission frequencies that arespaced apart in a transmission frequency range and said region includeselectromagnetic noise that can vary within said region and across saidtransmission frequency range, a portable device comprising: a receiverhaving a receiver bandwidth that at least includes said transmissionfrequency range and configured for operation in (i) a setup mode formeasuring the electromagnetic noise at least in said transmissionfrequency range to establish a frequency content of the electromagneticnoise for use in selecting one of the discrete transmission frequenciesas a selected transmission frequency that is subsequently utilized asthe electromagnetic locating signal during the operational procedure and(ii) in a locating mode for receiving the selected transmissionfrequency to provide certain information relating to saidelectromagnetic locating signal.
 43. A method for use in conjunctionwith a system in which a transmitter is moved through the ground in aregion during an operational procedure while transmitting a transmittersignal and said region includes electromagnetic noise that can varywithin said region and based on frequency, said method comprising: priorto said operational procedure, detecting the electromagnetic noise insaid region at an above ground location; determining a predicted maximumoperational depth of the transmitter for reception of said transmittersignal at the above ground location based, at least in part, on thedetected electromagnetic noise; and indicating the predicted maximumoperational depth at least prior to the operational procedure.
 44. Themethod of claim 43 wherein said transmitter signal is transmittableduring said operational procedure at a transmission frequency andwherein determining the predicted maximum operational depth includesestablishing an average value of the electromagnetic noise at least atthe transmission frequency and, thereafter, establishing a thresholdsignal value for the transmitter signal based at least in part on theaverage value for use in establishing the predicted maximum operationaldepth.
 45. The method of claim 44 wherein determining the predictedmaximum operational depth includes establishing a standard deviation ofthe electromagnetic noise and using the standard deviation inconjunction with the average value to establish the predicted maximumoperational depth.
 46. The method of claim 43 wherein said transmittersignal is transmittable at one of a plurality of different frequenciesduring the operational procedure and wherein said determining includesestablishing the maximum operational depth of the transmitter for eachone of the transmitter frequencies and said indicating includesdisplaying the maximum operational depth for each one of the pluralityof different frequencies.
 47. The method of claim 43 wherein saidtransmitter signal is transmittable by said transmitter during saidoperational procedure including modulation data and said method furthercomprising transmitting a simulated transmission signal that ismodulated by a simulated modulation data and said detecting includesreceiving the simulated transmission signal along with theelectromagnetic noise, and determining the predicted maximum operationaldepth includes modifying the simulated transmission signal to identifythe predicted maximum operational depth based on decodability of thesimulated modulation data.
 48. The method of claim 47 wherein thesimulated transmission signal includes a simulation transmissionfrequency and wherein modifying the simulated transmission signalincludes changing a signal strength of the simulated transmissionfrequency for a fixed value of the simulated transmission frequency toidentify a threshold signal strength that represents a minimum signalstrength at which the simulated modulation data is decodable such thatsaid minimum signal strength defines the predicted maximum operationaldepth.
 49. The method of claim 48 wherein changing the signal strengthof the simulated transmission frequency includes incrementallydecreasing the signal strength beginning initially from a value at whichthe simulated modulation data is decodable.
 50. An apparatus for use inconjunction with a system in which a transmitter is moved through theground in a region during an operational procedure while transmitting atransmitter signal and said region includes electromagnetic noise thatcan vary within said region, said apparatus comprising: a detector fordetecting the electromagnetic noise in said region at an above groundlocation prior to said operational procedure such that theelectromagnetic noise is detected without the transmitter signal; aprocessor configured for determining a predicted maximum operationaldepth of the transmitter for reception of said transmitter signal at theabove ground location based on the detected electromagnetic noise; and adisplay for indicating the predicted maximum operational depth at leastprior to the operational procedure.